This article is from the Pregnancy Screening FAQ, by Lynn Gazis-Sax (firstname.lastname@example.org) with numerous contributions by others.
Here are a list of references from the medical literature on human
population studies which examine the risks and benefits of ultrasound
screening during pregnancy. These references show that there is
presently very good evidence that no short or long-term effects result
from prenatal exposure to ultrasound. However, ultrasound exposure at
high intensities and long exposure times can cause problems in
laboratory animals. (see refs. (1, 2), for example.) Most of these
problems appear to be due to the heating which can result from long,
high intensity ultrasound. Thus, these human population studies have
been conducted to see whether such effects occur at the exposure times and
intensities actually used in clinical prenatal ultrasound. None of the
studies listed below specifically address vaginal ultrasound exams or
long-term exposure to Doppler ultrasound.
Several large human population studies have been performed in which
thousands to tens of thousands of women in low-risk pregnancies are
assigned randomly to either control groups or routinely-exposed groups.
The control groups are assigned to receive no routine ultrasound exams,
while the routinely-exposed groups are routinely examined, regardless of
need. Women in either group are given ultrasound exams if a problem
arises which makes the exam medically necessary, but no women are moved
between groups after their initial assignments. (This ensures that
women who develop problems during pregnancy aren't steadily moved from
the control to the routinely-exposed group. Were this the case, it
would not be surprising that the incidence of problems would increase in
the routinely-exposed group.) All but one of the large studies show no
evidence of harm, even for the large populations studied. (see refs (4,
7) and references therein.) The study by Newnham et al. (ref. 6) did
see an increase in the percentage of low-birthweight babies in an
intensively-examined group versus a control group receiving a very low
average number of exams; apart from this difference, the two groups had
identical pregnancy outcomes. However, similarly conducted studies
(refs. 4, 7) examined much larger populations, and found no problems.
Newnham also notes that the average difference in birthweights between
the two groups was very small, so that the finding could have been an
accident due to small numbers of cases.
Some studies don't bear out advantages from routine ultrasound for
low-risk pregnancies (see refs. 4,6). These studies are always designed
to exclude women who already have indications that their pregnancies
will present problems, such as unexplained bleeding, the assumption
being that ultrasound exams offer a net benefit for pregnancies with
known risk factors. However, some physicians think that these studies
underestimate the benefits of routine ultrasound; objections such as
these are aired in the correspondence following (refs. 4 and 6).
Most of the population studies listed below only follow women through
pregnancy and birth, but some are of long enough term to study children
past infancy (refs. 5,8,9) These are again reassuring, although the
numbers of children followed is much smaller than in the studies of
infants immediately after birth. One study (ref. 3) received wide media
coverage because it purported to show an association between hearing
loss and ultrasound exams. The authors studied a condition, delayed
speech, in a group of Canadian children. This condition is not known to
be caused by physiological problems, and may be a problem with
psychological origins. In their study, they compared a group of
children with this condition with a group of children of the same size
without delayed speech. This matching of the two groups was performed
after the fact, rather than by following two groups of children from
birth and watching them potentially develop this problem. The numbers
of children studied were very small (under one hundred, compared to the
much larger numbers examined in every other study mentioned here), and
the researchers didn't control for numerous other factors. Other major
problems in this study are detailed in the correspondence following the
article, in which the authors actually state that they do not consider
their work to show a link between ultrasound and delayed speech.
References on obstetrical diagnostic ultrasound and studies of
1. Barnett, G.R. ter Haar, M.C. Ziskin, W.L. Nyborg, K. Maeda, J. Bang,
"Current status of research on biophysical effects of ultrasound,"
Ultrasound in Medicine and Biology, vol. 20 (1994) pp. 205-218.
2. Bioeffects and Safety of Diagnostic Ultrasound, American Institute
of Ultrasound in Medicine, Rockville, Maryland, 1993.
3. Campbell, R.W. Elford, R.F. Brant, "Case-control study of prenatal
ultrasonography exposure in children with delayed speech," Canadian
Medical Association Journal, vol. 149 (Nov. 15, 1993), pp. 1435-40; Also
read the related correspondence in the same journal, vol. 150 (March 1,
1994), pp. 647-9.
4. Ewigman, J.P. Crane, F.D. Frigoletto, "Effect of Prenatal Ultrasound
Screening on perinatal outcome," New England Journal of Medicine, Vol.
329, pp. 821-7, 1993.
5. Lyons, C. Dyke, and M. Toms, "In utero exposure to diagnostic
ultrasound: a six year followup," Radiology, vol. 166 (1988) pp. 687-690.
6. Newnham, John P., Sharon F. Evans, Con A. Michael, Fiona J.
Stanley, and Louis I. Landau, "Effects of frequent ultrasound during
pregnancy: a randomised controlled trial," The Lancet, Vol. 342,
October 9, 1993, pp. 887-891; see also related correspondence in the
same journal, Nov. 27, 1993, pp. 1359-61 and Jan. 15, 1994, p. 178.
7. Saarri-Kemppainen, O. Karjalainen, P. Ylostalo, O.P. Heinonen,
"Ultrasound screening and perinatal mortality: controlled trial of
systematic one-stage screening in pregnancy. The Helsinki Ultrasound
Trial," The Lancet, vol. 336 (1990), pp. 387- 391.
8. Scheidt, F. Stanley, D.A. Bryla, "One year follow-up of infants
exposed to ultrasound in utero," American Journal of Obstetrics and
Gynecology, vol. 131 (1978), pp. 743- 748.
9. Stark, M. Orleans, A.D. Havercamp, "Short and long-term risks after
exposure to diagnostic ultrasound in utero." Obstetrics and Gynecology,
vol. 63 (1984) pp. 194- 200.